The formation of ice upon surfaces such as machinery, walkways and buildings has long been a problem when these surfaces are exposed to environments conducive to icing. For example, the effect of ice formations on the critical surfaces of aircraft can lead to severely altered aerodynamic characteristics. This can result in the inability of the aircraft to operate safely during takeoff and flight. The reduction of aerodynamic properties is evident from both wind tunnel and flight tests in which the leading edge and upper surface of wings were provided with ice formations having a thickness and roughness similar to that of sandpaper. Such icing has been found to reduce wing lift by as much as 30% and to increase drag by approximately 40%. These changes can severely reduce aircraft flight characteristics and controllability. The process of ascertaining airworthiness (i.e., a clean aircraft free of ice formations prior to takeoff) requires a team effort with the pilot making the final judgement that the aircraft is in a condition for safe flight.
Currently, the only known method for positively determining that an aircraft is free of any ice formations prior to takeoff is by close, visual inspection of its surface. A practice in use by some flight operators is to view from the cabin or cockpit critical surface areas of the aircraft (i.e., leading edges, wing surfaces and engine inlets) for indication of ice formations. In particular, recent accidents involving both large transport aircraft as well as small general aviation aircraft indicate that the current standards and methods upon which air-worthiness is based are often inadequate. Moreover, during inclement weather conditions (i.e., heavy precipitation, high wind velocity, and low ambient temperature), an observer's ability to visually note the appearance of solid phase ice domains in a partially liquid surface coating the material (i.e., a roadway, automobile, or aircraft) is often compromised.
The use of freezing point depressant (FPD) fluids to aid in ground de-icing processes and to provide a protective film to delay the formation of ice, frost and snow prior to takeoff is a method currently used by the aviation industry, as well as in treating outdoor surfaces such as walkway, roadways and driveways. U.S. Pat. No. 3,928,654 (A. J. Bonnanzio).
Despite the ability of FPD fluids to provide de-icing to critical surfaces, a need still exists for an accurate determination of the effectiveness of de-icing fluids on the critical surfaces of, for example, roadways, and machinery such as aircraft. Factors such as ambient temperature, surface temperature, concentration of de-icing fluid, rate of precipitation, and wind velocity which are difficult to instantaneously ascertain, influence the rate at which the de-icing fluids lose their effectiveness and make quantitative judgements of the condition of these surfaces exposed to low temperature environments difficult to obtain.
In particular, current visual observation methods for ascertaining ice formation on aircraft surfaces are not adequate when weather conditions are inclement and the volume of traffic is such that aircraft may be treated with de-icing fluids and then forced to wait for extended periods prior to take-off. During the period prior to takeoff, precipitation and other factors can cause de-icing fluids to become diluted, thereby reducing their FPD ability. This effect allows the formation of solid phase domains on the aircraft surface. According to FAA policy, in order to have an effective anti-icing solution on the surface prior to take-off, the freezing point of residual fluids (FPD and mixtures) should be at least about 20.degree. below the ambient temperature or the surface temperature of the aircraft, whichever is less. However, with the addition of precipitation onto the aircraft prior to take-off, the freeze point of the liquid present on the aircraft can quickly approach the environmental temperature, thereby resulting in zones of icing.
Thus, a need exists for a de-icing method which would provide flight crews or ground personnel with a simple means for determining presence of ice, frost and snow on critical aircraft surfaces.